1. Field of the Invention
This invention relates to a brazing process and more particularly to a brazing process applied to nickel-base and nickel-iron-base superalloys to obtain satisfactory joints between parts made of such materials.
2. The Prior Art
It has become increasingly important, especially in high temperature aircraft applications, such as, for example, in turbine engine parts, to use materials for structural applications that are capable of withstanding high temperatures and corrosive attacks normally associated therewith. Precipitation hardenable alloys such as nickel-base superalloys are nickel-iron-base alloys have been employed, where possible, to meet requirements of high strength-to-weight ratios, corrosion resistance, etc. at elevated temperatures. The greatest impediment to efficient use of these materials, however, has been the difficulty of obtaining satisfactory joints between parts made of such alloy materials. Brazing is normally the means whereby joints between such parts are created. Brazing consists of joining base metal surfaces by fusing (generally above 850.degree. F.) a braze material, having a lower melting point than the subject base metal, without appreciable fusion of the base metal surfaces themselves. The brazing process depends on capillary action to draw the molten braze material into a controlled gap and wetting of the mating base metal surfaces by the molten braze material. A chief cause of brazement defects is the presence of oxides on the base metal surfaces which inhibit the flow and wetting of the molten braze material over the subject base metal surfaces. The oxide surface problem is particularly acute with precipitation hardened nickel-base and nickel-iron based superalloys in which precipitation strengthening elements such as aluminum and titanium are included in the alloy. Elements such as aluminum and titanium are relatively reactive metals and during heating to the brazing or bonding temperature readily form oxides on the base metal surface. As already discussed, the presence of these surface oxides interferes with effective brazing operations. One approach to preventing oxide build-up is to pre-plate the base metal surfaces with a very thin layer (0.0006 inch) of nickel onto the areas of the base metal to be joined prior to application thereto of the braze material and heating. However, this approach has not been entirely successful because frequently the areas to be joined are not readily accessible to nickel plating because of the joint configuration; the nickel plating is susceptible to damage during handling and assembly of the parts to be brazed and the nickel plating often will remain partially unmelted during the brazing cycle when the joint is brazed with a low melting point braze material, and the nickel plating may not properly adhere to and react with the base metal during brazing if cleaning of the base metal prior to plating is not scrupulously performed.
In recent years, significant accomplishments have been reported in vacuum hot press, solid-state diffusion bonding of superalloys such as those based on nickel. However, the presence of surface oxides also interferes with diffusion bonding of precipitation hardened nickel base and nickel-iron base superalloys. Because of the sensitivity of diffusion bonding to surface oxide formation, the base metal surfaces are nickel-phosphorus plated to prevent formation of undesirable surface oxides. Diffusion bonding of nickel-phosphorus plated parts commonly produces welded joints characterized by poor mechanical properties as compared to the base metal, the welded joint being the weakest link in the structure. The reduced mechanical properties have been attributed to the lack of chemical homogeneity in the joint region as the result of the presence of nickel-phosphorus plating layer.
It is therefore a major object of the present invention to provide an improved method for preparing the surface of a nickel-base or nickel-iron base superalloy for brazing or diffusion bonding and characterized as overcoming the above described problems.
It is a further object of the present invention to provide a method of surface preparation of precipitation hardened nickel-base or nickel-iron base superalloy substrate surfaces for brazing or diffusion bonding wherein the use of nickel plating or nickel phosphorus plating is eliminated.